The TFT semiconductor device has used a heatproof insulating substrate made of quartz or the like. TFTs have been fabricated at a high density by performing a high-temperature process above 1000.degree. C. Such TFT semiconductor devices have been earnestly developed, for example, as active matrix array substrates for active matrix liquid crystal displays. To realize wider application of liquid crystal displays, there is a demand for reductions in the cost of fabricating TFT semiconductor devices. Low-temperature processes with which cheap glass substrates can be used have been discussed. Especially, in order to fabricate a large-sized, high-information content liquid crystal display, low-temperature processes capable of utilizing cheap glass substrates have been earnestly developed. In this connection, a technique consisting of forming a film of amorphous silicon on a low-melting point glass substrate and irradiating the film with a laser beam to convert the film into high-quality polysilicon has been studied. Since polysilicon has a larger carrier mobility than that of amorphous silicon, high-performance TFTs can be formed at a high density.
However, where amorphous silicon is used as a precursory film of clusters, even if the silicon is irradiated with a laser beam, it is impossible to obtain sufficiently large grain size. Rather, only crystallites are obtained. With silicon thin films in the state of crystallites, it is difficult to obtain TFTs of high performance. However, if the energy of the irradiating laser beam is optimized to some extent, it is possible to convert amorphous silicon into polysilicon. Also in this case, the range of the irradiating energy enabling satisfactory grain sizes is very narrow and so it is difficult to stably mass-produce high-quality polysilicon thin films in practice. It is common practice to form amorphous silicon thin films by plasma-assisted CVD. However, the films contain a large amount of hydrogen. For this reason, the contained hydrogen is caused to bump by the laser irradiation, and other problems occur. These make it difficult to put this technique into practical use.